Simple sugars may be categorized as ketoses or aldoses. When ketoses and aldoses undergo ring closure, they form hemiketals and hemiacetals, respectively. In a hemiketal or hemiacetal, the "carbonyl" carbon is attached to a ring carbon and a ring oxygen. The incorporation of the "carbonyl" carbon into the ring structure of the hemiketal or hemiacetal imparts a new chirality, according to the configuration of the substituents. Accordingly, the "carbonyl" carbon of an aldose or a ketose is termed the "anomeric" carbon.
An oxygen linked glycosyl bond is formed when an anomeric carbon of an hemiacetal or hemiketal is condensed with an alcohol to form a ketal or acetal. The condensation reaction creates a glycosyl bond between the anomeric carbon and the alcohol. If both the hemiacetal or hemiketal and the alcohol are monosaccharides, then the resultant acetal or ketal is a disaccharide. The two subunits of the disaccharide are linked through at least one anomeric carbon by a glycosyl bond.
In an acetal, the anomeric carbon is located at a terminal position on the carbon backbone of the aldose. Accordingly, bond linkages with the anomeric carbon of the acetal include a ring carbon, a ring oxygen, a glycosyl oxygen, and a hydrogen.
In a ketal, the anomeric carbon is centered at a non-terminal carbon within the backbone of the ketose. Accordingly, bond linkages with the anomeric carbon of the ketal include a ring carbon, a ring oxygen, a glycosyl oxygen, and a carbon side chain.
Glycosyl bonds may be formed with or without the assistance of enzyme catalysis. Most natural products having glycosyl bonds are formed with the assistance of enzyme catalysis. A broad array of naturally occurring oligosaccharides and polysaccharides are included within this category. Unfortunately, many oxygen linked glycosyl products are subject to enzymic degradation.
Martin et al. (Tetrahedron Letters (1992), 33 (41), pp 6123-6126) disclose a non-enzymic method for forming oxygen linked glycosyl bonds. Martin discloses the use of diethyl phosphite as a leaving group for synthesizing oxygen linked glycosyl bonds.
What is needed is an easily synthesized glycosyl bond which can serve as an analog to oxygen linked glycosylation bonds but which does not employ oxygen as a linkage unit.